1. Field of the Invention
The present invention relates to a novel device and system for the monitoring of the moisture content of soil.
2. Description of the Prior Art
Many fruit crops, particularly citrus, show a correlation between their resistance to frost damage in winter and their hydration states of the previous summer. Devices which would be used to monitor commercial orchards to provide the farmer with information, particularly during times of drought, with an instant reading on the hydration state of the trees would be extremely valuable to the industry. Such information would not only allow the farmer to schedule irrigation to conserve water and use it more efficiently, but would allow the farmer to "tune" the hydration state of his orchard in summer to produce the desired frost-resistance for the following winter.
Information concerning the water content of large trees is also invaluable to commercial companies that specialize in transplanting large trees. These companies usually warrant the transplanted tree for periods ranging up to one year after the transplant. The act of transplanting a large tree creates considerable stress on the tree due to the injury or removal of large portions of the tree's root system. This usually hinders the tree's ability to remove water out of the soil, thus making the tree very susceptible to drying out. Therefore, an inexpensive and easy way of monitoring the tree's hydration state can allow the caretakers to give a greater amount of attention to those trees in need and would provide an economical way of verifying that the customer did not neglect the trees during the warranty period.
Although knowledge of the moisture status of the soil-plant system is essential for the study of plant water relations, techniques for the determination of the water content of both soils and plant tissues continue to be questioned [Gardner, "Water content: an overview," Int. Conf. on Measurement of Soil and Plant Water Status, Utah State University, Logan, Utah, 1:7-9 (1987); and Kramer, "Plant relative water content and related methods: Historical perspectives and current concerns," Int. Conf. on Measurement of Soil and Plant Water Status, Utah State University, Logan, Utah, 2:1-8 (1987)]. Existing techniques for measurement of plant moisture content can be criticized as insensitive, inaccurate or indirect. What is required is an inexpensive, readily automated, portable technique that is sensitive to changes in water content while being insensitive to the character of the matrix [Gardner, supra].
Stem water contents are difficult to determine because of high levels of hydration, geometrical constraints and potentially deleterious effects of intrusions into living tissues. Changes in stem diameter [Hinckley et al, "Temporal and spatial variations in the water status of forest trees," Forest Science Monograph 20 (1978)], extraction of tissue cores [Waring et al, "Sapwood water storage: its contribution to transpiration and effect upon water conductance through the stems of old-growth Douglas-fir," Plant, Cell and Environment, Vol. 1, pages 131-140 (1978); and Waring et al, "The contribution of stored water to transpiration in Scots pine," Plant Cell and Environment, Vol. 2, pages 309-317 (1979)], tissue water potential [Goldstein et al, "Influence of insulating dead leaves and low temperatures on water balance in an Andean giant rosette plant," Plant Cell and Environment, Vol. 6, pages 649-656 (1983); and Nobel et al, "Transpiration stream of desert species: resistances and capacitances for a C.sub.3, a C.sub.4 and a CAM plant," Journal of Experimental Botany, Vol. 34, pages 1379-1391 (1983)], and gamma ray attenuation [Edwards et al, "A method for measuring radial differences in water content of intact tree stems by attenuation of gamma radiation," Plant, Cell and Environment, Vol. 6, pages 255-260 (1983); and Brough et al, "Diurnal changes in water content of the stems of apple trees, as influenced by irrigation," Plant, Cell and Environment, Vol. 9, pages 1-7 (1986)] have all been used to monitor changes in stem moisture content. Dimensional changes are both easily automated and non-destructive, but are sensitive only to water content changes in the extra-cambial elastic region of the stem. In the case of palms, the existence of a relatively stiff outer layer surrounding the living, elastic tissues [Tomlinson, "Anatomy of the Monocotyledons: I. Palmae" (1961)] renders this technique unsatisfactory. Stem tissue, extracted using an increment borer, samples the entire cross-section, but the technique is destructive and may alter the water content of the sample due to tissue compression [Holbrook, "The role of stem water storage in the arborescent palm, Sabal palmetto," M. Sc. Thesis, University of Florida, Gainesville, Fla. (1989)]. Psychrometers inserted into stems have several disadvantages, including local tissue damage, release of cell contents into the apoplast during insertion, temperature fluctuations and calibration difficulties. Gamma ray attenuation avoids many of these problems, but its application is complicated by safety considerations.
Another approach to monitoring tissue moisture status is to measure the dielectric constant. The dielectric constant (.epsilon.) is an intrinsic property of a material and relates to the ability of a material to store electrical energy reversibly. The dielectric constant (.epsilon.) is defined as: EQU .epsilon.=1+X (1)
where X is the electrical susceptibility--the proportionality constant between the electric field and the degree of polarization [Jackson, Classical Electrodynamics, John Wiley & Sons, New York (1975)]. Because of its large dipole moment and ability to form hydrogen bonds, water has an extremely high dielectric constant (78.3 at 25.degree. C.) compared to most solids and liquids (3 to 10) [Wheast, Handbook of Chemistry and Physics, CRC Press, Cleveland, Ohio (1975)]. Furthermore, the dielectric properties of pure water are fairly insensitive to temperature (approximately -0.37.degree. C..sup.-1 from 10.degree. to 30.degree. C.) [Wheast, supra] and independent of frequency up to 10.sup.10 Hz [Jackson, supra]. Although solutes do affect the dielectric properties of an aqueous medium, at low concentrations and high frequencies this influence is thought to be small [Hasted, Aqueous Dielectrics, Chapman and Hall, London (1973)]. Changes in the apparent dielectric constant of a water-permeated medium, therefore, may result from primarily changes in the moisture content [Sheriff, "An apparatus for the measurement of leaf dielectric properties in the high frequency region," Journal of Experimental Botany, Vol. 27, pages 645-650 (1976); Topp et al, "Electromagnetic determination of soil water content: measurements in coaxial transmission lines," Water Resources Research, Vol. 16, pages 574-582 (1980); and Pissis et al, "A dielectric study of the state of water in plant stems," Journal of Experimental Botany, Vol. 38, pages 1528-1540 (1987)].
Dielectric measurements can be made in both the frequency and time-domain [Hasted, supra]. Both approaches have been used to determine plant and soil moisture with varying degrees of success [Sheriff, supra; Dalton et al, "Time-domain reflectometry: simultaneous measurement of soil water content and electrical conductivity with a single probe, "Science, Vol. 224, pages 989-990 (1984); Halbertsma . et al, "Application and accuracy of a dielectric soil water content meter," Proc. Int. Conf. Measurement Soil Plant Water Status, 1:11-15 (1987); Topp, "The application of time-domain reflectometry (TDR) to soil water content measurement," Proc. Int. Conf. Soil Plant Water Status, 1:85-93 (1987); and Harbinson et al, "The use of microwaves to monitor the freezing and thawing of water in plants," Journal of Experimental Botany, Vol. 38, pages 1325-1335 (1987)].
Sheriff [supra] uses a primitive radio-frequency oscillator (about 25 mHz) to measure the capacitance of a capacitor formed by two plates with a leaf and some remaining air forming a dielectric layer. Sheriff then measured the effective dielectric constant of this layered system and calculated (knowing the textbook value for air being 1) the dielectric constant of his leaf. Sheriff's apparatus uses two plates of fixed position as opposed to plates designed to be completely in contact with the plant. The apparatus measures the total capacitance of the capacitor plates. This includes their capacitance coupling to objects outside the plates. This means that zeroing the apparatus is necessary before use and not altering the local environment (e.g., placement of nearby laboratory objects) during measurements is critical. For example, the movement of a hand near the apparatus while taking a measurement would result in the apparatus "seeing" the hand as well as the leaf under study. This has been a nuisance that has restricted the wide application of this technique.
U.S. Pat. No. 4,114,090 to Poskitt does form a type of dielectric capacitor with the sample (in this case, tobacco leaves) forming the dielectric between the capacitor plates. This capacitor forms part of an oscillator circuit whose frequency depends on the capacitance of the capacitor (10) and on the value of the resistors (11 and 12). However, since the patent does not describe how the oscillator works, one cannot assess whether the device will, in addition to responding to the dielectric constant of the leaves (assumed to be due to moisture), give erroneous readings due to changes in the conductivity of the leaves. Actually, the aforementioned patents are either obviously susceptible to giving erroneous readings due to conductivity changes in the materials under test or the patent does not give enough details to determine whether the circuit will give erroneous readings. Conductivity changes would usually come about in the materials due to changes in ion concentrations ("saltiness") due to evaporation of water, due to the chemistry of the material, or due to its history. For example, sponge soaked in salt water has a high conductivity (about 10.sup.-1 mhos/cm), one soaked in distilled water a low conductivity (about 10.sup.-6 mhos/cm). Yet they have the same dielectric constant and so all devices using dielectric measurements to read moisture content should give the same reading. A truly valuable device would be one that is not fooled by the conductivity difference.
Sheriff and Poskitt were using dielectric sensing which is susceptible to errors due to influences of objects near, but not in, the "sensing" region of the capacitor sensors due to fringe fields. This has been another traditional problem associated with all types of capacitance sensors.
There is also a need for devices and systems for measuring the moisture content of soil, particularly at specified depths. Systems presently available for carrying out these methods suffer from several serious drawbacks.
Like the above-described plant hydration detection systems, presently available soil moisture detection systems are also sensitive to external influences. Moreover, they are influenced by the salt content of the soil, yielding inconsistent hydration measurements depending on the ion profile in the soil.
One such system, a "Capacitance Soil Moisture Probe," marketed by Didcot Instrument Co., Ltd. of England (Oxford), is limited in its applicability by the fact that it operates in the radio frequency (RF) region at 150 mHz and does not provide means for shielding from external influences.
It is an object of the present invention to provide a system for monitoring the moisture content of soil which is not subject to the above-noted prior art disadvantages.
More particularly, it is an object of the present invention to provide such a system which is able to actively suppress the fringe fields for the inner "sensing" region of the sensors, i.e., a system which would ensure that external influences such as people or animals in contact with the soil would not give false readings.